The session_id column should always be indexed for speedy lookups.
Session data is marshaled to the data column in Base64 format.
If the data you write is larger than the column's size limit,
ActionController::SessionOverflowError will be raised.

You may configure the table name, primary key, and data column.
For example, at the end of config/application.rb:

Note that setting the primary key to the session_id frees you from
having a separate id column if you don't want it. However, you must
set session.model.id = session.session_id by hand! A before filter
on ApplicationController is a good place.

Since the default class is a simple Active Record, you get timestamps
for free if you add created_at and updated_at datetime columns to
the sessions table, making periodic session expiration a snap.

You may provide your own session class implementation, whether a
feature-packed Active Record or a bare-metal high-performance SQL
store, by setting